Systems and methods for ion implantation of semiconductor wafers

Abstract

A dual mechanical movement scanning system for ion implantation includes a radial scan arm which is mounted at a fixed pivot point external to the system vacuum chamber and penetrates the chamber and supports the wafer paddle or wheel within the chamber. The fast-scan component is provided by rotation of the paddle on the support arm; the slow-scan component is provided by pivotal movement of the scan arm itself. The drive system which reciprocally pivots the radial scan arm is configured geometrically so that controlled movement of one arm of the drive system is translated as l/r velocity of the rotational axis relative to the beam path. The wafer paddle can be constructed to provide a variable implant angle and to use a component of centrifugal force to hold the wafers in place without clamps or other mechanical means.

What is claimed is: 
     
       1. In a system for scanning semiconductor wafers through an ion beam, means generating an ion beam oriented in a prearranged direction defining one axis of a rectangular coordinate system relative thereto; a scan wheel assembly for carrying a plurality of semiconductor wafers and having a central axis; drive means for rotating said scan wheel assembly about said central axis to scan said wafers across said ion beam generally in the direction of a second axis of said rectangular coordinate system; scan means for producing relative scanning movement between said scan wheel assembly and said ion beam generally in the direction of a third axis of said rectangular coordinate system; said scan wheel assembly comprising a central hub, a plurality of separate spoke arms mounted to and supported by said central hub and extending radially outward therefrom; and a plurality of heat sink elements, and said heat sink elements being supported on said spoke arms substantially without intervening structure; each supported on the outer end of one of said spoke arms and including means for mounting a semiconductor wafer thereon, said spoke arms each having a width substantially less than the maximum width of said heat sink elements; said scan means overscanning said heat sink elements in and out of said ion beam so that said ion beam strikes only a portion of said spoke arms in one overscan position and the total heat load produced on said scan wheel assembly by said ion beam is substantially reduced. 
     
     
       2. The system of claim 1, wherein said drive means comprises a mounting drive means for mounting said scan wheel assembly with said central axis generally parallel to the direction of said ion beam but canted at a slight angle toward the origin of said ion beam in a plane defined by said central axis and said one axis and for rotating said scan wheel assembly at high rotational velocity about said central axis to scan said wafers across said ion beam in one coordinate direction; each of said heat sink elements defining a mounting surface region and being spaced by an associated spoke arm at a prearranged distance from said central axis; each of said wafer mounting means comprising a heat sink insert mounted to said heat sink element and defining a wafer mounting surface for carrying a prearranged size of semiconductor wafer thereon and being oriented at a prearranged angle relative to said one axis such that a line normal to said wafer mounting surface in said plane defined by said central axis and said one axis intersects said central axis to define a large acute angle, whereby rotation of said scan wheel assembly at said high rotational velocity produces a substantial component of centrifugal force normal to said wafer mounting surface so that a wafer thereon is held tightly against said mounting surface when said scan wheel assembly is rotating at high rotational velocity. 
     
     
       3. The system of claim 2, wherein each of said heat sink elements defines a wafer edge restraint adjacent said heat sink insert at the outermost radial point thereon to restrain a wafer carried on said wafer mounting surface when said scan wheel assembly is in rotation; and further comprising wafer clamping means mounted on said heat sink element for temporarily urging a wafer carried on said wafer mounting surface against said wafer edge restraint when said scan wheel assembly is not rotating at high rotational velocity. 
     
     
       4. The system of claim 3, wherein said wafer clamping means includes a pair of clamp finger means, means mounting said clamp finger means to said heat sink element on a side opposite said wafer mounting surface and includng spring means urging said clamp finger means against an edge portion of a wafer carried on said wafer mounting surface and inertial means opposing said spring means when said scan wheel assembly is rotating at high velocity to retract said clamp finger means away from said edge portion of said wafer so that said wafer is retained on said wafer mounting surface solely by said component of centrifugal force normal to said wafer. 
     
     
       5. The system of claim 2, wherein said central axis of said scan wheel assembly is canted at an angle A relative to a line parallel to said one coordinate axis; each of said spoke arms having an outer arm portion canted with respect to said central hub by an angle B in the direction of the origin of said ion beam where B is equal to A plus 3.5 degrees; and each of said heat sink inserts has a first surface mounted on said heat sink element and a second surface comprising said wafer mounting surface defined at an angle of 3.5 degrees to said first surface; said heat sink inserts being selectably mountable on said heat sink element in one of a first orientation with said 3.5-degree angle additive to angle B to place said wafer mounting surface at an angle of 7 degrees to said ion beam or a second orientation with said 3.5-degree angle subtractive from angle B to place said wafer mounting surface at an angle of 0 degrees to said ion beam. 
     
     
       6. The system of claim 2, wherein each of said heat sink elements defines a cooling fluid channel therethrough; said scan wheel assembly includes piping means coupling cooling fluid to and from each of said cooling fluid channels in said heat sink assemblies to carry heat generated by said ion beam away therefrom; and each of said heat sink inserts comprises a conductive metal body defining a first mounting surface adjacent said heat sink assembly and a second mounting surface facing said ion beam, a first layer of conductive elastomeric material mounted on said first mounting surface, and a second layer of conductive elastomeric material mounted on said second mounting surface to serve as said wafer mounting surface, whereby said heat sink insert provides good thermal conductivity to said cooled heat sink assembly across the entire surface of a wafer mounted thereon when said scan wheel assembly is rotated at high velocity to produce said component of centrifugal force urging said wafer against said second layer of conductive elastomeric material. 
     
     
       7. The system of claim 6, wherein each of said heat sink elements defines a wafer edge restraint adjacent said heat sink insert at the outermost radial point thereon to restrain said wafer on said wafer mounting surface when said scan wheel assembly is in rotation; and further comprising wafer clamping means mounted on said heat sink element for temporarily urging a wafer carried on said wafer mounting surface against said wafer edge restraint when said scan wheel assembly is not rotating at high rotational velocity; said wafer clamping means includes a pair of clamp finger means, means mounting said clamp finger means to said heat sink element on a side opposite said wafer mounting surface and including spring means urging said clamp finger means against an edge of a wafer carried on said wafer mounting surface and inertial means opposing said spring means when said scan wheel assembly is rotating at high velocity to retract said clamp finger means away from said edge of said wafer so that said wafer as retained on said wafer mounting surface solely by said component of centrifugal force normal to said wafer. 
     
     
       8. The system of claim 1, further comprising a vacuum chamber for housing said scan wheel assembly and having front and back walls generally parallel to the plane of said scan wheel assembly, wherein said drive means and said scan means comprise a combined mounting drive means for mounting said scan wheel assembly in said vacuum chamber with said central axis oriented substantially parallel to said one axis, for rotating said scan wheel assembly about said central axis, and for translating said scan wheel assembly relative to said ion beam substantially in the direction of a second axis of said rectangular coordinate system to produce a two dimensional scan of said semiconductor wafer through said ion beam; said mounting drive means comprising a scan arm disposed within said vacuum chamber, first mounting means for mounting said scan wheel assembly at one end of said scan arm for rotation about said central axis, first drive means associated with said first mounting means for rotationally driving said scan wheel assembly, second mounting means for mounting said scan arm at the other end thereof for rotation about an axis generally parallel to said one axis including shaft means attached at one end to said scan arm and extending through said front wall of said vacuum chamber, bearing means external to said vacuum chamber for journaling said shaft means for rotation, second drive means associated with said shaft means for rotating said shaft in said bearing means, and vacuum sealing means cooperatively mounted between said wall of said vacuum chamber and said shaft means to provide a rotary vacuum-to-air shaft sealing arrangement. 
     
     
       9. The system of claim 8, wherein said mounting drive means mounts said scan wheel assembly with said central axis generally parallel to the direction of said ion beam but canted at a slight angle toward the origin of said ion beam in a plane defined by said central axis and said one axis; each of said heat sink elements defining a mounting surface region and being spaced by an associated spoke arm at a prearranged distance from said central axis; each of said wafer mounting means comprising a heat sink insert mounted to said heat sink element and defining a wafer mounting surface for carrying a prearranged size of semiconductor wafer thereon and being oriented at a prearranged angle relative to said one axis such that a line normal to said wafer mounting surface in said plane defined by said central axis and said one axis intersects, said central axis to define a large acute angle, whereby rotation of said scan wheel assembly at said high rotational velocity produces a substantial component of cenrifugal force normal to said wafer mounting surface so that a wafer thereon is held tightly against said mounting surface during said rotational scanning. 
     
     
       10. The system of claim 9, wherein each of said heat sink elements defines a wafer edge restraint adjacent said heat sink insert at the outermost radial point thereon to restrain a wafer carried on said wafer mounting surface when said scan wheel assembly is in rotation; and further comprising wafer clamping means mounted on said heat sink element for temporarily urging a wafer carried on said wafer mounting surface against said wafer edge restraint when said scan wheel assembly is not rotating at high rotational velocity. 
     
     
       11. The system of claim 10, wherein said wafer clamping means includes a pair of clamp finger means, means mounting said clamp finger means to said heat sink element on a side opposite said wafer mounting surface and including spring means urging said clamp finger means against an edge of a wafer carried on said wafer mounting surface and inertial means opposing said spring means when said scan wheel assembly is rotating at high velocity to retract said clamp finger means away from said edge of said wafer so that said wafer is retained on said wafer mounting surface solely by said component of centrifugal force normal to said wafer. 
     
     
       12. The system of claim 9, wherein said central axis of said scan wheel assembly is canted at an angle A relative to a line parallel to said one axis; each of said spoke arms having an outer arm portion canted with respect to said hub by an angle B in the direction of the origin of said ion beam where B is equal to A plus 3.5 degrees; and each of said heat sink inserts has a first surface mounted on said heat sink element and a second surface comprising said wafer mounting surface defined at an angle of 3.5 degrees to said first surface; said heat sink inserts being selectably mountable on said heat sink element in one of a first orientation with said 3.5-degree angle additive to angle B to place said wafer mounting surface at an angle of 7 degrees to said ion beam or a second orientation with said 3.5-degree angle subtractive from angle B to place said wafer mounting surface at an angle of 0 degrees to said ion beam. 
     
     
       13. The system of claim 9, wherein each of said heat sink elements defines a cooling fluid channel therethrough; said scan wheel assembly includes piping means coupling cooling fluid to and from each of said cooling fluid channels in said heat sink assemblies to carry heat generated by said ion beam away therefrom; and each of said heat sink inserts comprises a conductive metal body defining a first mounting surface adjacent said heat sink assembly and a second mounting surface facing said ion beam, a first layer of conductive elastomeric material mounted said first mounting surface, and a second layer of conductive elastomeric material mounted on said second mounting surface to serve as said wafer mounting surface, whereby said heat sink insert provides good thermal conductivity to said cooled heat sink assembly across the entire surface of a wafer mounted thereon when said scan wheel assembly is rotated at high velocity to produce said component of centrifugal force urging said wafer against said second layer of conductive elastomeric material. 
     
     
       14. The system of claim 9, wherein each of said heat sink elements defines a wafer edge restraint adjacent said heat sink insert at the outermost radial point thereon to restrain said wafer on said wafer mounting surface when said scan wheel assembly is in rotation; and further comprising wafer clamping means mounted on said heat sink element for temporarily urging a wafer on said wafer mounting surface against said wafer edge restraint when said scan wheel assembly is not rotating at high rotational velocity; said wafer clamping means includes a pair of clamp finger means, means mounting said clamp finger means to said heat sink element on a side opposite said wafer mounting surface and including spring means urging said clamp finger means against an edge of a wafer carried on said wafer mounting surface and inertial means opposing said spring means when said scan wheel assembly is rotating at high velocity to retract said clamp finger means away from said edge of said wafer so that said wafer is retained on said wafer mounting surface solely by said component of centrifugal force normal to said wafer. 
     
     
       15. In a system for scanning semiconductor wafers through an ion beam, a vacuum chamber; means generating an ion beam and directing said ion beam into said vacuum chamber in a prearranged direction defining one axis of a rectangular coordinate system; a scan wheel assembly for carrying a plurality of semiconductor wafers and having a central axis; mounting drive means for mounting said scan wheel assembly in said vacuum chamber with said central axis oriented substantially parallel to said one axis, for rotating said scan wheel assembly about said central axis, and for translating said scan wheel assembly relative to said ion beam substantially in the direction of a second axis of said coordinate system to produce a two dimensional scan of said semiconductor wafer through said ion beam; said mounting drive means comprising a scan arm disposed within said chamber, first mounting means for mounting said scan wheel assembly at one end of said scan arm for rotation about said central axis, first drive means associated with said first mounting means for rotationally driving said scan wheel assembly, second mounting means for mounting said scan arm at the other end thereof for rotation about an axis generally parallel to said one axis including shaft means attached at one end to said scan arm and extending through a wall of said vacuum chamber, bearing means external to said vacuum chamber for journaling said shaft means for rotation, second drive means associated with said shaft means for rotating said shaft in said bearing means, and vacuum sealing means cooperatively mounted between said vacuum chamber wall and said shaft means to provide a rotary vacuum to air shaft sealing arrangement. 
     
     
       16. The system of claim 15, wherein said axis of said ion beam I, said central axis of said scan wheel assembly A and said axis of said second mounting means B intersect a common orthogonal plane at points which define a triangle BAI having two fixed sides BA and BI and a third side AI which varies in length as said scan arm scans back and forth across said beam; said second drive arrangement comprising means for driving said scan arm so that the rate of change of the distance AI varies inversely with the magnitude of the distance AI, including a pivot arm mounted at one end to said shaft and a linear drive means mounted for rotation about an axis E and attached to the other end of said pivot arm at a point D so that said linear drive means translates said other end of said pivot arm toward said axis E; the positions of said axis E and said point of attachment D being preselected such that said points B, D, and a point on said axis E coplanar with points B, D define a triangle BDE which is congruent to said triangle BAI; said second drive arrangement further including tracking means for signalling the distance DE as said linear drive means moves said pivot arm, and means for controlling the rate of drive of said linear drive means as a function of the inverse of said signalled distance DE so that the rate of change of the distance DE varies inversely with the magnitude of the distance DE and, because of triangular congruency, the corresponding rate of change of the distance AI varies inversely with the magnitude of the distance AI. 
     
     
       17. In a system for scanning semiconductor wafers through an ion beam, means generating an ion beam oriented in a prearranged direction defining one axis of a rectangular coordinate system relative thereto; a scan wheel assembly for carrying a plurality of semiconductor wafers and having a central axis; mounting drive means for mounting said scan wheel assembly with said central axis generally parallel to the direction of said ion beam but canted at a slight angle toward the origin of said ion beam in a plane defined by said central axis and said axis and for rotating said scan wheel assembly at high rotational velocity about said central axis to scan said wafers across said beam generally in the driection of a second axis of said coordinate system; said scan wheel assembly including wafer mounting means comprising structural means defining a plurality of separate mounting surface regions disposed at radially arrayed locations spaced a prearranged distance from said central axis and a wafer support assembly mounted at each of said mounting surface regions and defining a flat wafer mounting surface having a prearranged angle relative to said one axis such that a line normal to said wafer mounting surface in said plane defined by said central axis and said one coordinate axis intersects said central axis to define a large acute angle, whereby rotation of said scan wheel assembly at said high rotational velocity produces a substantial component of centrifugal force normal to said wafer mounting surface so that a wafer thereon is held tightly against said mounting surface during said rotational scanning. 
     
     
       18. In a method for scanning semiconductor wafers through an ion beam, the steps of: disposing a plurality of generally cylindrical heat sinks radially around a central hub with narrow spokes connecting said heat sinks to said central hub and rotatably mounting said hub on a scan arm itself mounted for pivotal movement about a selected point spaced from the hub;   mounting a plurality of individual wafers on said heat sink elements;   rotating said heat sinks and wafers around said central hub at a high velocity; and   pivoting said scan arm to scan said heat sinks and wafers back and forth through said beam with a velocity which varies proportional to 1/r where r is the distance between said ion beam and said central hub.   
     
     
       19. In a method for scanning semiconductor wafers through an ion beam, the steps of: disposing a plurality of wafer mounting surfaces on a scan wheel with each at a common prearranged first angle to a major plane of said scan wheel;   mounting said scan wheel for rotation about an axis which is canted toward the origin of said ion beam direction by a small acute angle which is of value equal to or less than said first angle; and   rotating said scan wheel about said axis at high angular velocity to provide a component of centrifugal force normal to said wafer surface even when said second angle is equal to said first angle.   
     
     
       20. In a method for scanning semiconductor wafers through an ion beam, the steps of: mounting said wafers on prearranged radially arrayed positions on a scan wheel;   mounting said scan wheel for rotation about a central axis thereof on one end of a scan arm;   mounting the other end of said scan arm for rotation about a scan axis generally parallel to said central axis and generally parallel to said ion beam;   mounting a pivot arm at one end to said scan arm to drive said scan arm about said scan axis; and   translating the other end of said pivot arm toward a fixed point in space which, together with said other end of pivot arm and a point on said scan axis coplanar therewith, defines a triangle which is congruent with a triangle defined by points on said scan axis, said central axis and said ion beam in a plane mutually orthogonal thereto, said translating being at a velocity which is inversely proportional to the distance between said other end of said pivot arm and said fixed point so that said central axis of said scan wheel translates toward said ion beam with a velocity between said central axis and said ion beam which is inversely proportional to the distance therebetween because of said congruency.   
     
     
       21. In a system for scanning a semiconductor wafer through an ion beam, a scan wheel;   means mounting said scan wheel for rotation about a central axis generally parallel to said ion beam;   means for driving said scan wheel to a high rotational velocity in a plane generally parallel to the direction of gravitational forces;   a heat sink element carried on said scan wheel and defining a mounting surface for carrying a semiconductor wafer on said heat sink, said mounting surface being disposed at a prearranged angle to said central axis such that a line normal to said mounting surface intersects said central axis to form a small acute angle;   restraining means mounted adjacent said mounting surface for restraining a wafer carried thereon from flying off said mounting surface due to a high value component of centrifugal force on said wafer in the plane of said wafer during high rotational velocity of said scan wheel; and   means for temporarily retaining said wafer on said mounting surface while said scan wheel is at rest or is spinning up toward high rotational velocity, said wafer being held tightly against said mounting surface by a component of centrifugal force normal to said wafer during high rotational velocity of said scan wheel.   
     
     
       22. The system of claim 21, wherein said restraining means includes at least one wafer clamping means comprising clamp finger means, mounting means mounting said clamp finger means adjacent said wafer mounting surface including spring means urging said clamp finger means into contact with an edge portion of a wafer carried on said mounting surface in the absence of centrifugal force on said mounting means and inertial means responsive to centrifugal force on said mounting means during high rotational velocity of said scan wheel to overcome said spring means and thereby to retract said clamp finger means away from said edge portion of said wafer. 
     
     
       23. In a system for scanning semiconductor wafers through an ion beam, means generating an ion beam oriented in a prearranged direction defining one axis of a rectangular coordinate system relative thereto; a scan wheel assembly for carrying a plurality of semiconductor wafers and having a central axis; drive means for rotating said scan wheel assembly about said central axis to scan said wafers accross said ion beam generally in the direction of a second axis of said rectangular coordinate system; scan means for producing relative scanning movement between scan wheel assembly and said ion beam generally in the direction of a third axis of said rectangular coordinate system; said scan wheel assembly comprising a central hub, a plurality of separate spoke arms mounted to and supported by said central hub and extending radially outward therefrom; and a plurality of heat sink elements, each supported on the outer end of one of said spoke arms and including means for mounting a semiconductor wafer thereon, said spoke arms each having a width substantially less than the maximum width of said heat sink elements; said scan means overscanning said heat sink elements in and out of said ion beam so that said ion beam strikes only a portion of said spoke arms in one overscan position and the total heat load produced on said scan wheel assembly by said ion beam is substantially reduced; and wherein said drive means comprises a mounting drive means for mounting said scan wheel assembly with said central axis generally parallel to the direction of said ion beam but canted at a slight angle toward the origin of said ion beam in a plane defined by said central axis and said one axis and for rotating said scan wheel assembly at a high rotational velocity about said central axis to scan said wafers accross said ion beam in one coordinate direction; each of said heat sink elements defining a mounting surface region and being spaced by an associated spoke arm at a prearranged distance from said central axis; each of said wafer mounting means comprising a heat sink insert mounted to said heat sink element and defining a wafer mounting surface for carrying a prearranged size of semiconductor wafer thereon and being oriented at a prearranged angle relative to said one axis such that a line normal to said wafer mounting surface in said plane defined by said central axis and said one axis intersects said central axis to define a large acute angle, whereby rotation of said scan wheel assembly at said high rotational velocity produces a substantial component of centrifugal force normal to said wafer mounting surface so that a wafer thereon is held tightly against said mounting surface when said scan wheel assembly is rotating at high rotational velocity.